1. Field of the Invention
The present invention relates to an internal combustion engine with multiple intake valves for each cylinder for improvement of combustion.
2. Description of the Related Art
Known in the prior art is an internal combustion engine with two intake valves for each cylinder. Such two-valve engines are disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 52-340108, 54-84128, 58-172424, and 58-1725514. These have a first intake port of a helical shape, a second intake port of a straight shape, and a fuel injector mounted in the first intake port. This type of engine is aimed at providing a stratified combustible mixture in the combustion chamber for achieving a leaner burn.
An intake control valve can be further provided in the second port with the fuel injector located between the intake valve and the intake control valve, as disclosed in Japanese Patent Application Nos. 58-228329 and 59-69176 filed by the same assignee of the present case. In this case, the valve operating timing is set such that the second intake port is opened later than the first intake port. When the intake control valve is closed, air only is first induced in the combustion chamber through the first port and then subsequently the fuel is induced. Therefore, good stratification of the mixture is obtained, with the mixture lean near the piston and rich near the cylinder head where the spark plug is located. This enables good combustion with an overall lean mixture. In this design, it is preferably to set the opening timing of the second intake valve later than the first intake valve.
There is one problem with this arrangement. Since the second valve is open at a later stage of the suction stroke, less air is induced through the second port when the intake control valve is open.